# Is it possible to slow light down?

13 December 2016

## Question

I have read that light travelling through a Bose-Einstein condensate may be slowed down to about 38mph. This is a big drop from 186000 miles per second. How does it happen?

We asked physicist Kerstin Goepfrich if light can ever be slowed down...

Kerstin - Well, light travels at the speed of about 7.5 times around the Earth in just a second. That's really very fast and that is the speed at which light travels.

Why? Well light is made of a unit of energy called the photon. And the photon has energy, it has a momentum but it doesn't have mass, and physics tells us that an object without mass travels at the speed of light. So that's true for a photon and that's also true for gravitational waves, for instance.

But it can take longer for light to travel a given distance. So how can that be if it always travels at the speed of light? Well, in simplified picture, you can imagine that light that's travelling at the speed of light, hits an atom where it gets absorbed, then it takes a while for it to get re-emitted, then it travels at the speed of light again until it hits the next atom, and so on and so forth. That is why it can take light longer to travel through a medium like water, for instance as opposed to travelling through a vacuum.

Chris - Because it does physically slow down when it goes from one medium into a more optically dense medium, doesn't it, like glass, like water?

Kerstin - And when you say optically dense, that is exactly what a refractive index means. The factor by which light slows down, when it for instance goes through water, is the factor which we call refractive index so, for instance water has a refractive index of 1.3, which means the light is 1.3 times faster when it travels through a vacuum compared to water.

Chris - So the bottom line is that you can slow light down and it does, all the time, speed up and slow down when it goes from one medium into another medium. That's why it bends and refracts and that's why, when you look over your toaster at the wall behind your toaster when you're making your toast in the morning, the wall looks wavy and wiggly because the light is bouncing off the wall and speeding, up slowing down and bending.

Kerstin - Even though it is still travelling at the speed of light. And if you want to measure that there is actually a simple experiment that you can do with your Christmas chocolate if you've got some left over. You put it into the microwave because microwaves are also travelling at the speed of light, and you know the frequency at which your microwave operates, you can look at the two points where your chocolate melts and measure the distance between them. If you google microwave and speed of light then you'll probably find the details and instructions on how to do it.

Chris - In fact, you could go to nakedscientists.com/kitchenscience because we have put there the worked experiment of how you measure the speed of light with your microwave. And we did by buttering lots of bread and melting patches of butter and because the wavelength of a microwave and an oven is about twelve and a half centimeters or so. So you get melting every time there's a peak or trough on the wave so that's every six centimetres, so you can actually work out the speed of light because c speed of light = f the frequency x lambda the wavelength. And you can look on the back of the microwave and it will tell you the frequency of the microwave is 2.45GHz.

Giles - Well this sounds like a lot more fun than playing with a puzzle after you've finished you turkey. I think this sounds like we should have that in our Christmas dinner this year.